Engines may be configured with direct fuel injectors (DI) for injecting fuel directly into an engine cylinder and/or port fuel injectors (PFI) for injecting fuel into an intake port of an engine cylinder. Fuel injectors often have piece-to-piece and time-to-time variability due to imperfect manufacturing processes and/or injector aging, for example. Over time, injector performance may degrade (e.g., injector becomes clogged) which may further increase piece-to-piece injector variability. As a result, the actual amount of fuel injected to each cylinder of an engine may not be the desired amount and the difference between the actual and desired amounts may vary between injectors. Such discrepancies may lead to reduced fuel economy, increased tailpipe emissions, and an overall decrease in engine efficiency. Further, engines operating with a dual injector system, such as dual fuel or PFDI systems, may have even more fuel injectors (e.g., twice as many) resulting in greater possibility for degradation of engine performance due to injector degradation.
Diverse approaches may be used to estimate the variability in injector performance. One example approach is shown by Pursifull et al. in US20150240739. Therein, a port injector is calibrated by pressurizing both a low pressure and a high pressure fuel rail by operating a low pressure and a high pressure pump, suspending operation of both the pumps simultaneously, and then fueling a single cylinder via port injection while the remaining cylinders are fueled via direct injection. After each port injection event, a pressure decrease in the low pressure fuel rail coupled to the port injector is measured and compared to a predetermined value. Any deviation between the measured pressure drop and the expected pressure drop based on the injected fuel quantity is learned and compensated.
However, the inventors herein recognized potential issues with the above approach. As one example, the approach of Pursifull may not be accurate across all fuel pulse-widths and fuel temperatures. The inventors have herein recognized that port fuel injection accuracy is not only dependent on the injection pressure, but also on injection temperature. The injector temperature not only affects the injector resistance, but also affects the fuel density. Due to the effect of temperature on injector resistance, an injector offset may vary. Injector offset results from the difference in injector opening time and injector closing time. If injector opening delay and closing delay were identical and otherwise symmetric, injector offset would be negligible. However, injector opening is governed by the supply voltage, injector resistance, and injection pressure (for a given injector design and fuel condition). Injector closing is governed by a distinct set parameters. In addition to the injector offset, due to the effect of temperature on the fuel density, the commanded fuel mass and the injected fuel mass may vary, causing further discrepancies between fuel mass commanded on a port injection event and a pressure drop measured at the port injection event. Consequently, as injector temperature changes, as occurs during engine operation due to changing engine loads, the injector performance may vary. Due to the specific position of the port injectors, upstream of a combustion chamber and mounted at the back of a cylinder intake valve, port injector sensitivity to temperature variability may be exacerbated. As another example, the measured pressure drop following a port injection event may be inaccurate at lower fuel rail (or port injection) pressures as well as at lower port injection volumes, such as may occur at low load conditions. Specifically, the fuel quantity injected as a “percent of value” may have reduced accuracy as the fuel quantity or pulse width commanded to the port injector decreases. Likewise, at lower fuel rail pressures, there is a possibility of fuel vapor being ingested instead of liquid fuel, resulting in inaccurate pressure drops being measured. Inaccuracies in the pressure drop measurement may translate to inaccuracies in injector variability estimation. Fuel injector errors can result in air-fuel ratio discrepancies in cylinders, leading to misfires, reduced fuel economy, increased tailpipe emissions, and an overall decrease in engine efficiency.
In one example, the issues described above may be at least partially addressed by a method for an engine comprising: learning port injector variability as a function of injector current, the injector current estimated based on sensed port injection fuel rail temperature; and adjusting port fueling of the engine based on the learning. In this way, the injector offset can be corrected at all temperatures. In this way, the effect of temperature on injector variability may be better accounted for.
As an example, responsive to port fuel injection calibration conditions being met, port injector variability may be mapped as a function of injector voltage. This includes performing a predetermined number of port injection events from each port injector while sweeping injection voltage and while maintaining an injection pressure. At each injection event, a drop in fuel rail pressure may be measured and correlated to an initial parameter indicative of variability for the corresponding injector, thereby generating an initial injector offset map. To improve the accuracy of pressure drop measurement, particularly at low loads, a parallel pressure relief valve may be coupled to an inlet of the fuel rail so as to maintain the fuel rail pressure elevated (e.g., above fuel line pressure) even after the fuel pump is disabled. For each injection event, an injector temperature may be inferred from fuel rail temperature, as sensed via a temperature sensor coupled to the port injection fuel rail. Injector resistance may be calculated based on the inferred injector temperature and the injection voltage. The initial injector offset map which correlates injector variability to injector voltage may then be transformed into an updated injector offset map which relates injector variability to injector current.
In this way, port fuel injector variability may be accurately determined over a wide range of temperature conditions. The technical effect of learning a fuel injection quantity correction as a function of injector voltage, and then mapping it to a function of injector current is that injector variability due to variations in injector or fuel temperature can be accounted for. By using an existing fuel rail temperature sensor to infer injector temperature, the need for dedicated temperature sensors is reduced, providing component and cost reduction benefits. By enabling a port injection fuel rail pressure to be held elevated above a fuel line pressure while a lift pump is disabled, it is possible to provide sufficiently large injection quantities to sustain an accurately measurable fuel rail pressure drop during port injector calibration. By enabling the port injector variability to be learned by running at any fuel pulse-width, the routine is rendered non-intrusive.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.